This application discloses an invention which is related, generally and in various embodiments, to a two-terminal modular multilevel converter (M2LC) subsystem, and a M2LC system including a plurality of M2LC subsystems (cells).
Many papers have been published regarding the Modular Multilevel Converter (M2LC) topology. FIGS. 1 and 2 illustrate different two-level configurations of a two-terminal M2LC cell. In many instances, the M2LC cells shown in FIGS. 1 and 2 are packaged as a single three-level M2LC cell having two terminals as shown in FIG. 3.
As shown in FIG. 1, the M2LC cell includes two switching devices (Q1 and Q2), two diodes, a capacitor (C1) and two terminals. With the configuration shown in FIG. 1, the two switching devices can be controlled such that one of two different potentials may be present across the two terminals of the M2LC cell. The two different potentials are (1) zero volts and (2) VC1 which is the voltage present on storage capacitor C1. If switching device Q2 is turned on, zero volts are present between the two terminals of the M2LC cell. If switching device Q1 is turned on, the voltage VC1 is present between the two terminals of the M2LC cell. It will be appreciated that in order to avoid short circuiting of the storage capacitor C1 and the significant damage likely to result therefrom, switching device Q1 should be off when switching device Q2 is on, and switching device Q2 should be off when switching device Q1 is on.
Similarly, as shown in FIG. 2, the M2LC cell includes two switching devices (Q3 and Q4), two diodes, a capacitor (C2) and two terminals. With the configuration shown in FIG. 2, the two switching devices can be controlled such that one of two different potentials may be present across the two terminals of the M2LC cell. The two different potentials are (1) zero volts and (2) VC2 which is the voltage present on storage capacitor C2. If switching device Q3 is turned on, zero volts are present between the two terminals of the M2LC cell. If switching device Q4 is turned on, the voltage VC2 is present between the two terminals of the M2LC cell. It will be appreciated that in order to avoid short circuiting of the storage capacitor C2 and the significant damage likely to result therefrom, switching device Q3 should be off when switching device Q4 is on, and switching device Q4 should be off when switching device Q3 is on.
As shown in FIG. 3, the three-level M2LC cell includes four switching devices (Q1, Q2, Q3 and Q4), four diodes, two capacitors (C1 and C2) and two terminals. It will be appreciated that capacitors C1 and C2 are typically identical for this arrangement. With the configuration shown in FIG. 3, the four switching devices can be controlled such that one of three different potentials may be present across the two terminals of the M2LC cell. The three different potentials are (1) zero volts, (2) VC1 which is the voltage present on storage capacitor C1 or VC2 which is the voltage present on storage capacitor C2, and (3) VC1+VC2 which is the sum of the voltages present on storage capacitors C1 and C2. Because the two storage capacitors C1 and C2 are typically sized the same, it will be appreciated that the voltages VC1 and VC2 are substantially identical, and the voltage VC1+VC2 is substantially identical to either 2VC1 or 2VC2.
For the M2LC cell of FIG. 3, if switching devices Q2 and Q3 are both turned on, zero volts are present between the two terminals of the M2LC cell. If switching devices Q1 and Q3 are both turned on, the voltage VC1 is present between the two terminals of the M2LC cell. If switching devices Q2 and Q4 are both turned on, the voltage VC2 is present between the two terminals of the M2LC cell. If switching devices Q1 and Q4 are both turned on, the voltage VC1VC2 is present between the two terminals of the M2LC cell. It will be appreciated that the independent control of the two voltage states VC1 and VC2 allow for the balancing of the charges on capacitors C1 and C2. It should also be apparent to those skilled in the art of this topology that the functionality of the M2LC cell of FIG. 3 may be realized by connecting the two-level M2LC cells of FIGS. 1 and 2 in series so that the emitter connection of the switching device Q2 of the two-level M2LC cell of FIG. 1 is connected to the collector connection of the switching device Q3 of the two-level M2LC cell of FIG. 2 if the switch functions applied to switching devices Q1, Q2, Q3, and Q4 are identical. The advantage of the M2LC cell of FIG. 3 is primarily packaging and minimization of control since it is possible for this M2LC cell to share a single controller (not shown) as opposed to two independent controllers required for each of the M2LC cells of FIGS. 1 and 2.
It will be appreciated that the M2LC topology possesses the advantages of the Cascaded H Bridge (CCH) topology in that it is modular and capable of high operational availability due to redundancy. Additionally, the M2LC topology can be applied in common bus configurations with and without the use of a multi-winding transformer. In contrast to M2LC, CCH requires the utilization of a multi-winding transformer which contains individual secondary windings which supply input energy to the cells.
However, unlike CCH, the M2LC cells (or subsystems) are not independently supplied from isolated voltage sources or secondary windings. For a given M2LC cell, the amount of energy output at one of the two terminals depends on the amount of energy input at the other one of the two terminals.
Multiple M2LC cells have previously been arranged in a traditional bridge configuration. For such configurations, the M2LC cells are arranged into two or more output phase modules, each output phase module includes a plurality of series-connected M2LC cells, and each output phase module is further arranged into a positive arm (or valve) and a negative arm (or valve), where each arm (or valve) is separated by an inductive filter. Each output phase module may be considered to be a pole. The outputs of the respective output phase modules may be utilized to power an alternating current load such as, for example, a motor.
Although the M2LC cell arrangements described hereinabove have proven to be useful, the arrangements are not necessarily optimal for all potential applications. Additionally, from a size and cost standpoint, utilizing two identical storage capacitors to realize the respective voltage states adds more size and cost to the M2LC cells than is necessary.